Sources of magnetic fields-magnetic monopoles-have so far proven elusive as elementary particles. Condensed-matter physicists have recently proposed several scenarios of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin-ice state is argued to be well described by networks of aligned dipoles resembling solenoidal tubes-classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defects look like magnetic monopoles. We demonstrated, by diffuse neutron scattering, the presence of such strings in the spin ice dysprosium titanate (Dy2Ti2O7). This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate the density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction.

Sodium cobaltate (Na(x)CoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.

NeXus is an effort by an international group of scientists to define a common data exchange and archival format for neutron, X-ray and muon experiments. NeXus is built on top of the scientific data format HDF5 and adds domain-specific rules for organizing data within HDF5 files, in addition to a dictionary of well defined domain-specific field names. The NeXus data format has two purposes. First, it defines a format that can serve as a container for all relevant data associated with a beamline. This is a very important use case. Second, it defines standards in the form of application definitions for the exchange of data between applications. NeXus provides structures for raw experimental data as well as for processed data.

Low-temperature neutron diffraction and NMR studies of field-induced phases in linarite are presented for magnetic fields H∥b axis. A two-step spin-flop transition is observed, as well as a transition transforming a helical magnetic ground state into an unusual magnetic phase with sine-wave-modulated moments ∥H. An effective J[over ˜]_{1}-J[over ˜]_{2} single-chain model with a magnetization-dependent frustration ratio α_{eff}=-J[over ˜]_{2}/J[over ˜]_{1} is proposed. The latter is governed by skew interchain couplings and shifted to the vicinity of the ferromagnetic critical point. It explains qualitatively the observation of a rich variety of exotic longitudinal collinear spin-density wave, SDW_{p}, states (9≥p≥2).

The complex interplay between the 3d and 4 f moments in hexagonal ErMnO 3 is investigated by magnetization, optical second harmonic generation, and neutron-diffraction measurements. We revise the phase diagram and provide a microscopic model for the emergent spin structures with a special focus on the intermediary phase transitions. Our measurements reveal that the 3d exchange between Mn 3+ ions dominates the magnetic symmetry at 10 K < T < T N with Mn 3+ order according to the Γ 4 representation triggering 4 f ordering according to the same representation on the Er 3+ (4b) site. Below 10 K the magnetic order is governed by 4 f exchange interactions of Er 3+ ions on the 2a site. The magnetic Er 3+ (2a) order according to the representation Γ 2 induces a magnetic reorientation (Γ 4 → Γ 2 ) at the Er 3+ (4b) and the Mn 3+ sites. Our findings highlight the fundamentally different roles the Mn 3+ , R 3+ (2a), and R 3+ (4b) magnetism play in establishing the magnetic phase diagram of the hexagonal RMnO 3 system.

Non-graphitic carbons (NGCs) represent the most abundant class of sp 2 -hybridized carbon, materials (coal char coal, activated carbon, etc.). These carbons consist of small graphene layer stacks possessing significant structural disorder in both the single graphene sheets and the stacking. In this study an advanced evaluation approach for wide-angle neutron scattering (WANS) was developed, based on the method introduced by Ruland and Smarsly in 2002. In particular, we elucidated if and how the enhanced WANS data quality and larger values of the modulus of the scattering vector s-range affect the accuracy and the values of the size and disorder parametersbeing fitting parameters by themselvesin comparison to wide-angle X-ray scattering (WAXS), which is usually performed by laboratory equipment. We find a reasonable agreement for the parameters L a and L c , that is, the lateral dimension and stack height, within the error bars, whereas for the disorder parameters different results for WAXS and WANS were found, the origin of which is discussed. Thus, this study addresses the general issue of how reliably microstructural parameters can be determined from WAXS/WANS by fitting simulated WAXS and WANS curves, which are quality-impaired by added Gaussian noise at different levels and cut-off at different s-values. From this analysis, we estimated the minimal data quality required for a reliable NGC microstructural analysis based on WAXS/WANS. As an important finding, these simulations show that typical, standard WAXS laboratory setups are sufficient to provide reliable values for the most relevant structural parameters. Furthermore, pairdistribution function (PDF) analyses were performed on WAXS data obtained from a synchrotron facility. Comparing PDF and WAXS/WANS fitting analysis suggests the presence of small highly ordered oligoaromatic domains embedded in the larger graphene sheets, questioning the classical view on the NGC microstructure.

The R 2 PdSi 3 intermetallic compounds have been reported to crystallize in a hexagonal AlB 2 -derived structure, with the rare earth atoms on the Al sites and Pd and Si atoms randomly distributed on the B sites. However, the intricate magnetic properties observed in the series of compounds have always suggested complications to the assumed structure. To clarify the situation, x-ray and neutron diffraction measurements were performed on the heavy rare earth compounds with R = Gd, Tb, Dy, Ho, Er, Tm, which revealed the existence of a crystallographic superstructure. The superstructure features a doubled unit cell in the hexagonal basal plane and an octuplication along the perpendicular c direction with respect to the primitive cell. No structural transition was observed between 300 and 1.5 K. Extended x-ray absorption fine structure (EXAFS) analysis as well as density functional theory (DFT) calculations were utilized to investigate the local environments of the respective atoms. In this paper the various experimental results will be presented and it will be shown that the superstructure is mainly due to the Pd-Si order on the B sites. A structure model will be proposed to fully describe the superstructure of Pd-Si order in R 2 PdSi 3 . The connection between the crystallographic superstructure and the magnetic properties will be discussed in the framework of the presented model.

We report a detailed single-crystal and powder neutron diffraction study of Co2TiO4 and Co2SnO4 between the temperatures 1.6 K and 80 K to probe their spin structures in the ground state. For both compounds the strongest magnetic intensity was observed for the (111)M reflection due to ferrimagnetic ordering, which sets in below TN = 48.6 K and 41 K for Co2TiO4 and Co2SnO4, respectively. An additional low intensity magnetic reflection (200)M was noticed in Co2TiO4 due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and the single-crystal neutron data of Co2TiO4 we noticed a significant broadening of the magnetic (111)M reflection, possibly results from the disordered character of the Ti and Co atoms on the B site. Practically, the same peak broadening was found for the neutron powder data of Co2SnO4. On the other hand, from our single-crystal neutron diffraction data of Co2TiO4 we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case it can be expected that competing Jahn-Teller effects act along different crystallographic axes can induce anisotropic local strain. In fact, for both ions Ti 3+ and Co 3+ the 2tg levels split into a lower dxy level and yields a higher two-fold degenerate dxz/dyz level. As a consequence, one can expect a tetragonal distortion in Co2TiO4 with c/a < 1, which could not significantly detected in the present work.